Polymer composition for pipes

ABSTRACT

A multimodal polymer composition for pipes is disclosed. The polymer is a multimodal polyethylene with a density of 0.930-0.965 g/cm 3 , an MFR 5  of 0.2-1.2 g/10 min, an M n  of 8000-15000, an M w  of 180-330×10 3 , and an M w /M n  of 20-35, said multimodal polyethylene comprising a low molecular weight (LMW) ethylene homopolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction, said HMW fraction having a lower molecular weight limit of 3500, and a weight ratio of the LMW fraction to the HMW fraction of (35-55):(65:45).

FIELD OF THE INVENTION

The present invention relates to a multimodal polymer composition forpipes and a pipe prepared thereof.

BACKGROUND OF THE INVENTION

Nowadays, pipes of polymer material are frequently used for variouspurposes, such as fluid transport, i.e. transport of liquid or gas, e.g.water or natural gas, during which the fluid can be pressurised.Moreover, the transported fluid may have varying temperatures, usuallywithin the temperature range from about 0° C. to about 50° C. Suchpressure pipes are preferably made of polyolefin plastic, usuallyunimodal ethylene plastic such as medium density polyethylene (MDPE;density: 0.930-0.942 g/cm³) and high density polyethylene (HDPE;density: 0.945-0.965 g/cm³). By the expression “pressure pipe” herein ismeant a pipe which, when used, is subjected to a positive pressure, i.e.the pressure inside the pipe is higher than the pressure outside thepipe.

Polymer pipes are generally manufactured by extrusion, or, to a smallerextent, by injection moulding. A conventional plant for extrusion ofpolymer pipes comprises an extruder, a nozzle, a calibrating device,cooling equipment, a pulling device, and a device for cutting or forcoiling-up the pipe.

The properties of such conventional polymer pipes are sufficient formany purposes, although enhanced properties may be desired, for instancein applications requiring high pressure resistance, i.e. pipes that aresubjected to an internal fluid pressure for a long and/or short periodof time. As examples of properties which it is desirable to improve maybe mentioned the processability, the impact strength, the modulus ofelasticity, the rapid crack propagation resistance, the slow crackgrowth resistance, and the design stress rating of the pipe.

SUMMARY OF THE INVENTION

It has now been discovered that a superior pressure pipe may be obtainedby preparing it from a specific, well defined type of multimodalpolyethylene. More particularly, the multimodal polyethylene should havea medium to high density, have a broad molecular weight distribution, acarefully selected ratio between its low molecular weight fraction andhigh molecular weight fraction, and include a comonomer in its highmolecular weight fraction only.

Thus, the present invention provides a multimodal polyethylenecomposition for pipes, which multimodal polyethylene has a density of0.930-0.965 g/cm³ and an MFR₅ of 0.2-1.2 g/10 min, characterised in thatthe multimodal polyethylene has an M_(n) of 8000-15000, an M_(w) of180-330×10³, and an M_(w)/M_(n) of 20-35, said multimodal polyethylenecomprising a low molecular weight (LMW) ethylene homopolymer fractionand a high molecular weight (HMW) ethylene copolymer fraction, said HMWfraction having a lower molecular weight limit of 3500, and a weightratio of the LMW fraction to the HMW fraction of (35-55): (65-45).

Other distinguishing features and advantages of the invention willappear from the following specification and the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

As stated above, the pressure pipe composition of the present inventionis made from a multimodal polyethylene. This is in contrast to prior artpolyethylene pipes which usually are made of unimodal polyethylene.

The “modality” of a polymer refers to the form of its molecular weightdistribution curve, i.e. the appearance of the graph of the polymerweight fraction as function of its molecular weight. If the polymer isproduced in a sequential step process, utilizing reactors coupled inseries and using different conditions in each reactor, the differentfractions produced in the different reactors will each have their ownmolecular weight distribution. When the molecular weight distributioncurves from these fractions are superimposed into the molecular weightdistribution curve for the total resulting polymer product, that curvewill show two or more maxima or at least be distinctly broadened incomparison with the curves for the individual fractions. Such a polymerproduct, produced in two or more serial steps, is called bimodal ormultimodal depending on the number of steps. In the following allpolymers thus produced in two or more sequential steps are called“multimodal”. It is to be noted here that also the chemical compositionsof the different fractions may be different. Thus one or more fractionsmay consist of an ethylene copolymer, while one or more others mayconsist of ethylene homopolymer.

By properly selecting the different polymer fractions and theproportions thereof in the multimodal polyethylene a pipe with interalia enhanced process-ability can be obtained.

The pressure pipe composition of the present invention is a multimodalpolyethylene, preferably a bimodal polyethylene. The multimodalpolyethylene comprises a low molecular weight (LMW) ethylene homopolymerfraction and a high molecular weight (HMW) ethylene copolymer fraction.Depending on whether the multimodal polyethylene is bimodal or has ahigher modality the LMW and HMW fractions may comprise only one fractioneach or include sub-fractions, i.e. the LMW may comprise two or more LMWsub-fractions and similarly the HMW fraction may comprise two or moreHMW sub-fractions. It is a characterising feature of the presentinvention that the LMW fraction is an ethylene homopolymer and that theHMW fraction is an ethylene copolymer, i.e. it is only the HMW fractionthat includes a comonomer. As a matter of definition, the expression“ethylene homopolymer” used herein relates to an ethylene polymer thatconsists substantially, i.e. to at least 97% by weight, preferably atleast 99% by weight, more preferably at least 99.5% by weight, and mostpreferably at least 99.8% by weight of ethylene and thus is an HDethylene polymer which preferably only includes ethylene monomer units.Moreover, the lower limit of the molecular weight range of the HMWfraction is 3 500, preferably 4000. This means that almost all ethylenecopolymer molecules in the multimodal polyethylene pipe composition ofthe invention have a molecular weight of at least 3500, preferably atleast 4000. The reason for this is that the presence of comonomer in theLMW fraction gives a pressure pipe with poor strength.

In the present invention it is further important that the proportions ofthe LMW and HMW fractions (also known as the “split” between thefractions) are selected properly. More particularly, the weight ratio ofthe LMW fraction to the HMW fraction should lie in the range(35-55):(65-45), preferably (43-51):(57-49), most preferably(43-48):(57-52). It is important that the split lies within theseranges, because if the proportion of the HMW fraction becomes to greatit results in too low strength values and if it is too low it results inan unacceptable formation of gels.

The molecular weight distribution, as defined by the ratio of the weightaverage molecular weight (M_(w)) to the number average molecular weight(M_(n)), i.e. M_(w)/M_(n), of the multimodal polyethylene is ratherbroad at the present invention and has a value of 20-35, preferably22-30. The reason for this is to obtain a pressure pipe with a desiredcombination of good processability and good strength. Further, thenumber average molecular weight, M_(n), has a value of 8 000-15 000,preferably 9 000-14 000, while the weight average molecular weight,M_(w), has a value of 180-330×10³, preferably 200-320×10³ (180-260×10³,preferably 200-250×10³, for an MD pipe material and 250-330×10³,preferably 280-320×10³, for an HD pipe material).

The melt flow rate (MFR), which is equivalent to the term “melt index”previously used, is another important property of the multimodalpolyethylene for pipes according to the invention. The MFR is determinedaccording to ISO 1133 and is indicated in g/10 min. The MFR is anindication of the flowability, and hence the process-ability, of thepolymer. The higher the melt flow rate, the lower the viscosity of thepolymer. The MFR is determined at different loadings such as 2.1 kg(MFR_(2.1); ISO 1133, condition D) or 5 kg (MFR₅; ISO 1133, conditionT). At the present invention the multimodal polyethylene has an MFR₅ of0.2-1.2 g/10 min, preferably 0.3-1.0 g/10 min.

Another characterising feature of the present invention is the densityof the multimodal polyethylene. For reasons of strength the density liesin the medium to high density range, more particularly in the range0.930-0.965 g/cm³. Preferably, lower densities of 0.937-0.942 g/cm³ areused for smaller diameter MD pressure pipes, while higher densities of0.943-0.955 g/cm³ are used for larger diameter HD pressure pipes. Thepressure pipes of medium density multimodal polyethylene are some-whatmore flexible than pressure pipes of high density multimodalpolyethylene and may therefore more easily be coiled into a roll. On theother hand it is possible to obtain pressure pipes of a higher designstress rating with high density multimodal polyethylene than with mediumdensity multimodal polyethylene.

It should be noted that the multimodal polymer composition of thepresent invention is characterised, not by any single one of the abovedefined features, but by the combination of all the features defined inclaim 1. By this unique combination of features it is possible to obtainpressure pipes of superior performance, particularly with regard toprocessability, rapid crack propagation (RCP) resistance, design stressrating, impact strength, and slow crack growth resistance.

The processability of a pipe (or rather the polymer thereof) may bedetermined in terms of the number of screw revolutions per minute (rpm)of an extruder for a predetermined output of pipe in kg/h, but also thesurface appearance of the pipe is then important.

The rapid crack propagation (RCP) resistance of a pipe may be determinedaccording to a method called the S4 test (Small Scale Steady State),which has been developed at Imperial College, London, and which isdecribed in ISO DIS 13477. According to the RCP-S4 test a pipe isdiameters. The outer diameter of the pipe is about 110 mm or greater andits wall thickness about 10 mm or greater. When determining the RCPproperties of a pipe in connection with the present invention, the outerdiameter and the wall thickness have been selected to be 110 mm and 10mm, respectively. While the exterior of the pipe is at ambient pressure(atmospheric pressure), the pipe is pressurised internally, and theinternal pressure in the pipe is kept constant at a pressure of 0.5 MPapositive pressure. The pipe and the equipment surrounding it arethermostatted to a predetermined temperature. A number of discs havebeen mounted on a shaft inside the pipe to prevent decompression duringthe tests. A knife projectile is shot, with well-defined forms, towardsthe pipe close to its one end in the so-called initiating zone in orderto start a rapidly running axial crack. The initiating zone is providedwith an abutment for avoiding unnecessary deformation of the pipe. Thetest equipment is adjusted in such a manner that crack initiation takesplace in the material involved, and a number of tests are effected atvarying temperatures. The axial crack length in the measuring zone,having a total length of 4.5 diameters, is measured for each test and isplotted against the set test temperature. If the crack length exceeds 4diameters, the crack is assessed to propagate. If the pipe passes thetest at a given temperature, the temperature is lowered successivelyuntil a temperature is reached, at which the pipe no longer passes thetest, but the crack propagation exceeds 4 times the pipe diameter. Thecritical temperature (T_(crit)) i.e. the ductile brittle transitiontemperature as measured according to ISO DIS 13477 is the lowesttemperature at which the pipe passes the test. The lower the criticaltemperature the better, since it results in an extension of theapplicability of the pipe. It is desirable for the critical temperatureto be around −5° C. or lower. A pressure pipe made of the multimodalpolymer composition according to the present invention preferably has anRCP-S4 value of −1° C. (minimum requirement for an MD PE80 pipe) orlower, more preferably −4° C. (minimum requirement for an HD PE80 pipe)or lower, and most preferably −7° C. (minimum requirement for an HDPE100 pipe) or lower.

The design stress rating is the circumferential stress a pipe isdesigned to withstand for 50 years without failure and is determined fordifferent temperatures in terms of the Minimum Required Strength (MRS)according to ISO/TR 9080. Thus, MRS8.0 means that the pipe is a pipewithstanding an internal pressure of 8.0 MPa gauge for 50 years at 20°C., and similarly MRS10.0 means that the pipe withstands an internalpressure of 10 MPa gauge for 50 years at 20° C. A pressure pipe made ofthe multimodal polymer composition according to the present inventionpreferably has a design stress rating of at least MRS8.0, and mostpreferably MRS10.0.

The impact strength is determined as Charpy Impact Strength according toISO 179. A pressure pipe made of the multimodal polymer compositionaccording to the present invention preferably has an impact resistanceat 0° C. of at least 10 kJ/m², more preferably at least 14 kJ/m², andmost preferably at least 15 kJ/m².

The slow crack propagation resistance is determined according to ISO13479:1997 in terms of the number of hours the pipe withstands a certainpressure at a certain temperature before failure. A pressure pipe madeof the multimodal polymer composition according to the present inventionpreferably has a slow crack propagation resistance of at least 1000 hrsat 4.0 MPa/80° C., and more preferably at least 500 hrs at 4.6 MPa/80°C.

The modulus of elasticity is determined according to ISO 527-2/1B. Apressure pipe made of the multimodal polymer composition according tothe present invention preferably has a modulus of elasticity of at least800 MPa, more preferably at least 950 MPa, and most preferably at least1100 MPa.

A pressure pipe made of the multimodal polymer composition of thepresent invention is prepared in a conventional manner, preferably byextrusion in an extruder. This is a technique well known to the skilledperson an no further particulars should therefore be necessary hereconcerning this aspect.

It is previously known to produce multimodal, in particular bimodal,olefin polymers, such as multimodal polyethylene, in two or morereactors connected in series. As instance of this prior art, mention maybe made of EP 517 868, which is hereby incorporated by way of referenceas regards the production of multimodal polymers.

According to the present invention, the main polymerisation stages arepreferably carried out as a combination of slurrypolymerisation/gas-phase polymerisation. The slurry polymerisation ispreferably performed in a so-called loop reactor. The use of slurrypolymerisation in a stirred-tank reactor is not preferred in the presentinvention, since such a method is not sufficiently flexible for theproduction of the inventive composition and involves solubilityproblems. In order to produce the inventive composition of improvedproperties, a flexible method is required. For this reason, it ispreferred that the composition is produced in two main polymerisationstages in a combination of loop reactor/gas-phase reactor. Optionallyand advantageously, the main polymerisation stages may be preceded by aprepolymerisation, in which case up to 20% by weight, preferably 1-10%by weight, more preferably 1-5% by weight, of the total amount ofpolymers is produced. The prepolymer is preferably an ethylenehomopolymer (HDPE). At the prepolymerisation all of the catalyst ispreferably charged into a loop reactor and the prepolymerisation isperformed as a slurry polymerisation. Such a prepolymerisation leads toless fine particles being produced in the following reactors and to amore homogeneous product being obtained in the end. Generally, thistechnique results in a multimodal polymer mixture through polymerisationwith the aid of a Ziegler-Natta or metallocene catalyst in severalsuccessive polymerisation reactors. Chromium catalysts are not preferredin connection with the present invention because of the high degree ofunsaturation they confer to the polymer. In the production of, say, abimodal polyethylene, which according to the invention is the preferredpolymer, a first ethylene polymer is produced in a first reactor undercertain conditions with respect to hydrogen-gas pressure, temperature,pressure, and so forth. After the polymerisation in the first reactor,the reaction mixture including the polymer produced is fed to a secondreactor, where further polymerisation takes place under otherconditions. Usually, a first polymer of high melt flow rate (lowmolecular weight, LMW) and with no addition of comonomer is produced inthe first reactor, whereas a second polymer of low melt flow rate (highmolecular weight, HMW) and with addition of comonomer is produced in thesecond reactor. As comonomer of the HMW fraction various alpha-olefinswith 4-8 carbon atoms may be used, but the co-monomer is preferablyselected from the group consisting of 1-butene, 1-hexene,4-methyl-1-pentene, and 1-octene. The amount of comonomer is preferablysuch that it comprises 0.4-3.5 mol %, more preferably 0.7-2.5 molt ofthe multimodal polyethylene. The resulting end product consists of anintimate mixture of the polymers from the two reactors, the differentmolecular-weight-distribution curves of these polymers together forminga molecular-weight-distribution curve having a broad maximum or twomaxima, i.e. the end product is a bimodal polymer mixture. Sincemultimodal, and especially bimodal, ethylene polymers, and theproduction thereof belong to the prior art, no detailed description iscalled for here, but reference is had to the above mentioned EP 517 868.

As hinted above, it is preferred that the multimodal polyethylenecomposition according to the invention is a bimodal polymer mixture. Itis also preferred that this bimodal polymer mixture has been produced bypolymerisation as above under different polymerisation conditions in twoor more polymerisation reactors connected in series. Owing to theflexibility with respect to reaction conditions thus obtained, it ismost preferred that the polymerisation is carried out in a loopreactor/a gas-phase reactor. Preferably, the polymerisation conditionsin the preferred two-stage method are so chosen that a comparativelylow-molecular polymer having no content of comonomer is produced in onestage, preferably the first stage, owing to a high content ofchain-transfer agent (hydrogen gas), whereas a high-molecular polymerhaving a content of comonomer is produced in another stage, preferablythe second stage. The order of these stages may, however, be reversed.

In the preferred embodiment of the polymerisation in a loop reactorfollowed by a gas-phase reactor, the polymerisation temperature in theloop reactor preferably is 92-98° C., more preferably about 95° C., andthe temperature in the gas-phase reactor preferably is 75-90° C., morepreferably 80-85° C.

A chain-transfer agent, preferably hydrogen, is added as required to thereactors, and preferably 350-450 moles of H₂/kmoles of ethylene areadded to the reactor producing the LMW fraction and 20-40 moles ofH₂/kmoles of ethylene are added to the reactor producing the HMWfraction.

As indicated earlier, the catalyst for polymerising the multimodalpolyethylene of the invention preferably is a Ziegler-Natta typecatalyst. Particularly preferred are catalysts with a high overallactivity as well as a good activity balance over a wide range ofhydrogen partial pressures. As an example hereof may be mentioned thecatalysts disclosed in EP 688794 and in FI 980788. Such catalysts alsohave the advantage that the catalyst (procatalyst and cocatalyst) onlyneeds to and, indeed, only should be added in the first polymerisationreactor.

Although the invention has been described above with reference to aspecified multimodal polyethylene, it should be understood that thismultimodal polyethylene may include various additives such as fillers,etc. as is known and conventional in the art. Further, the pipe made ofthe specified multimodal polyethylene may be a single-layer pipe or formpart of a multilayer pipe including further layers of other pipematerials.

Having thus described the present invention it will now be illustratedby way of non-limiting examples of preferred embodiments in order tofurther facilitate the understanding of the invention.

EXAMPLE 1

A pipe resin was produced by means of a three-step process in aprepolymerisation loop-reactor followed by first a loop-reactor and thena gas phase-reactor. The split was 2:42:56. No comonomer was used in thetwo consecutive loop-reactors, while 1-butene was used as comonomer inthe HMW-fraction produced in the gas phase-reactor in an amount suchthat the 1-butene comonomer content of the total resulting polymer was2.6% by weight. A Ziegler Natta type catalyst as disclosed in EP 688 794was used. The Mn Of the final polymer was found to be 8500 and the Mw200000. Mw/Mn thus was 23.5. The density was 941 kg/m³ (ISO 1183 D) andMFR₅ was 0.65 g/10 min. (ISO 1133, condition T). The processability wasmeasured using a Battenfeldt 1-90-30B extruder, which gave an output of730 kg/h at a screw speed of 158 rpm. The extruder head temperature was220° C. and the die temperature was 210° C. Under the same conditions aconventional unimodal polyethylene pipe resin (MDPE with a density of940 kg/m³ and an MFR₅ of 0,85 g/10 min) gave an output of 690 kg/h.

Physical test values were as follows:

E-modulus (ISO 527-2/1B) 840 MPa Impact strength at 0° C. (ISO 179) 16kJ/m² Pressure test on unnotched >5000 h at 10.0 MPa/20° C. 32 mm pipe(ISO 1167) >1000 h at 4.6 MPa/80° C. >5000 h at 4.0 MPa/80° C. Pressuretest on notched >5000 h at 4.0 MPa/80° C. 110 mm pipe (ISO 13479)RCP-resistance in the T_(crit) = −4° C. S4-test on 110 mm pipe

EXAMPLE 2

A pipe resin was produced using the same reactor configuration as usedin example 1. The split was 1:45:54. No comonomer was used in the twoconsecutive loop reactors, while 1-butene was used in the HMW-fractionproduced in the gas phase reactor in an amount such that the 1-butenecomonomer content of the total resulting polymer was 1.3% by weight. Thesame catalyst type was used as in example 1. The M_(n) of the finalpolymer was found to be 10500 and the M_(w) 285000. M_(w)/M_(n) thus was27. The density was 959 kg/m³ and MFR₅ was 0,35 g/10 min.

Physical test values were as follows:

E-modulus (ISO 527-2/1B) 1135 MPa Impact strength at 0° C. (ISO 179)13.7 kJ/m² Pressure test on unnotched 594 h at 12.4 MPa/20° C. 110 mmpipe (ISO 1167) >10000 h at 5.0 MPa/80° C. Pressure test on notched 1500h at 4.6 MPa/80° C. 110 mm pipe (ISO 13479) RCP-resistance in theT_(crit) = −7° C.; P_(crit) > 10 bar S4-test on 110 mm pipe

1. A multimodal polethylene composition for pipes, which multimodalpolyethylene has a density of 0.930-0.965 g/cm³ and an MFR₅ of 0.2-1.2g/10 min, chacterised in that the multimodal polyethylene has an M_(n)of 8000-15000, an M_(w) of 180-330×10³, and an M_(w)/M_(n) of 20-35,said multimodal polyethylene comprising a low molecular weight (LMW)ethylene homopolymer fraction polymerized in the presence of a ZieglerNatta catalyst and with the addition of 350-450 moles of H₂/kmoles ofethylene and a high molecular weight (HMW) ethylene copolymer fractionpolymerized in the presence of a Ziegler Natta catalyst said HMWfraction having a lower molecular weight limit of 3500, and a weightratio of the LMW fraction to the HMW faction of (35-55):(65-45).
 2. Amultimodal polymer composition as claimed in claim 1, wherein themultimodal polymer is a bimodal polyethylene produced by(co)polymerisation in at least two steps.
 3. A multimodal polymercomposition as claimed in claim 1, wherein the ethylene copolymer of theHMW fraction is a copolymer of ethylene and a comonomer selected fromthe group consisting of 1-butene, 1-hexene, 4-methyl-1-pentene, and1-octene.
 4. A multimodal polymer composition as claimed in claim 1,wherein the amount of comonomer is 0.43.5 mol % of the multimodalpolymer.
 5. A multimodal polymer composition according to claim 1,having a weight ratio of the LMW fraction to the HMW fraction of(43-51):(57-49).
 6. A multimodal polymer composition as claimed in claim1, wherein the multimodal polymer has an MFR₅ of 0.3-1.0 g/10 min.
 7. Amultimodal polymer composition as claimed in claim 1, wherein thepolymer is obtained by slurry polymerisation in a loop reactor of a LMWethylene homoplymer fraction, followed by gas-phase polymerisation of aMW ethylene copolymer fraction.
 8. A multimodal polymer composition asclaimed in claim 7, wherein the slurry polymerisation is preceded by aprepolymerisation step.
 9. A multimodal polymer composition as claimedin claim 8, wherein the polymer is obtained by prepolymerisation in aloop reactor, followed by slurry polymerisation in a loop reactor of aLMW ethylene homopolymer fraction, and gas-phase polymerisation of a HMWethylene copolymer fraction.
 10. A multimodal polymer composition asclaimed in claim 7, wherein polymerisation procatalyst and cocatalystare added to the first polymerisation reactor only.
 11. A pipecharacterised in that it is a pressure pipe comprising the multimodalpolymer composition according to claim 1, which pipe withstands apressure of 8.0 MPa gauge during 50 years at 20° C. (MRS8.0).
 12. A pipeas claimed in claim 11, wherein the pipe is a pressure pipe withstandinga pressure of 10 MPa gauge during 50 years at 20° C. (MRS10.0).
 13. Apipe as claimed in claim 11, wherein the pipe has a rapid crackpropagation (RCP)S4-value of −1° C. or lower.
 14. A pipe as claimed inclaim 13, wherein the pipe has a rapid crack propagation (RCP)S4-valueof −7° C. of lower.
 15. A multimodal polymer composition as claimed inclaim 8, wherein polymerisation procatalyst and cocatalyst are added tothe first polymerisation reactor only.
 16. A multimodal polymercomposition as claimed in claim 9, wherein polymerisation procatalystand cocatalyst are added to the first polymerisation reactor only.